I believe you mean him because it’s Subaki and while I know he’s kinda feminine looking he is still male.

Here we can see Niles’s self-sabotage in action. He’s insecure and that insecurity makes him withdraw back into his status quo of flings rather than take a step into the unknown of commitment, especially considering a forbidden relationship between a Hoshidan when they’re at war.
I also based the breakup from this fic: https://archiveofourown.org/works/19314568?view_adult=true
It’s pretty good at exploring their dynamic and issues.